It is conventional to employ a birefringent filter (sometimes denoted hereinafter as a "BRF") as a tuning element in a laser cavity. For example, U.S. Pat. No. 3,868,592, issued Feb. 25, 1975 to Yarborough, et al., teaches positioning a BRF in a laser cavity with the BRF's front surface oriented at the Brewster angle with respect to the incident laser beam. The BRF, so oriented, is intended to transmit only a selected primary frequency component of the laser beam (although, in practice, other frequency components may also undesirably be transmitted through secondary transmission sidebands). In order to tune the laser beam's frequency (i.e., to shift the frequency of the peak of the filter transmission function), the BRF is rotated about the axis perpendicular to its front surface U.S. Pat. No. 3,868,592 teaches that several "Brewster angle oriented" BRFs can be stacked together, in order to narrow the frequency width of the filter while maintaining sufficient separation between the successive orders of the filter to prevent oscillation at more than one frequency.
In order to improve the amplitude ratio between the primary and secondary sidebands of a BRF assembly, it has been proposed that additional Brewster surfaces be added by stacking several "Brewster angle oriented" glass plates with one or more "Brewster angle oriented" BRFs. For example, see: Holtom, et al., "Design of a Birefringent Filter for High-Power Dye Lasers," IEEE J. of Quantum Electronics, V. QE-10, No. 8, pp. 577-579 (1974); and Hodgkinson, et al., "Birefringent Filters for Tuning Flashlamp-Pumped Dye Lasers: Simplified Theory and Design," Applied Optics, V. I7, No. 12, pp. 1944-1948 (1978).
However, we have recognized that when a conventional BRF assembly is positioned in the cavity of a pulsed laser, reflections of the laser beam from the front and rear surfaces of the assembly components result in undesirable "satellite" pulses. Such satellite pulses have the same (or similar) frequency content as does the desired primary laser pulse, but are delayed relative to the primary pulse by integral multiples of the quantity t=2(T/c)(n.sub.eff /cosine E), where T is the optical component thickness, n.sub.eff is the optical component's effective refractive index, E is the internal angle between the propagation ray and the normal to the component's surface, and c is the speed of light in a vacuum, in the case that the primary pulse width is less than the round trip travel time of the beam in the optical component.
The parasitic satellite pulse problem arises where conventional assemblies of thin BRFs are used in tunable lasers for producing very short laser output pulses. To permit generation of very short output pulses, a BRF assembly must have a broad oscillation frequency bandwidth. In order to achieve this broad spectral width characteristic, conventional BRF assemblies have employed thin birefringent components and thus have been subject to the parasitic satellite pulse problem.
We have recognized that the parasitic satellite pulse problem exists even where an attempt is made to orient conventional BRF assemblies at the Brewster angle. Thus, where the conventional "Brewster angle oriented" BRF assembly is a conventional stack (including one or more thin BRFs and one or more glass plates), reflections from "Brewster" surfaces will result in undesirable satellite pulses.
Until the present invention, problem of undesired satellite pulses in tunable lasers for producing short output pulses had neither been appreciated nor solved. The present invention solves the satellite pulse problem in tunable lasers by employing an optically thick BRF assembly having a broad oscillation frequency bandwidth. The inventive BRF assembly permits generation of ultrashort pulses with a tunable laser, without undesired parasitic satellite pulses.